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Covert channels can be used to send information in a way that is hard to detect, and that violates a system's security policy. Some, of the many ways described in the literature of how to implement a covert channel are presented in this report. The research on covert channels started in the early seventies. At this time, the main issue for researchers was leakage of information between security levels when users with different security clearance accessed the same mainframe simultaneously. The subject has since then evolved from studying covert channels in mainframes towards studying covert channels in computer networks. The covert channel research field is rather diverse Most of the recently published papers concern the discovery of a new channel or the possible detection or removal of a previously published channel. However, there are a small number of papers covering more general methods for covert channel identification or mitigation. Due to the property of enabling leakage of information from systems with sensitive information, knowledge regarding covert channels is necessary for the Swedish Armed Forces. This is especially true when systems are increasingly interconnected-as they often are these days-since such interconnections often require that only information meant to be communicated is actually transmitted.Thus, for the Swedish Armed Forces, important research problems are mainly techniques and methods for preventing and detecting covert channels.